As is generally known, a felt is used to remove water from a wet paper web in the press part of a papermaking machine.
In the press part PP of a papermaking machine shown in FIG. 14, water is removed from a wet paper web WW proceeding between a pair of press rolls PR, using a single felt 10A. In the apparatus shown in FIG. 15, water is removed from a wet paper web WW pinched between two felts 10A in the press part PP. In the apparatus shown in FIG. 16, in which the press part PP comprises a press roll PR and a press shoe PS with a resin belt SB therebetween, water is removed from a wet paper web WW pinched between two felts 10A.
In each of the cases illustrated in FIGS. 14-16, the felt 10A is driven by the rotating press roll or rolls PR, and is compressed in the press part PP.
The general structure of a felt 10A is illustrated in FIG. 17. The felt 10A is endless, and comprises a base body 20A, and a fibrous assembly 30A connected to the base body 20A. The base body, which may be a woven fabric, imparts strength to the felt. The felt 10A enters into the press part PP in contact with a wet paper web, and is compressed as pressure is applied in the press part PP. The felt recovers its pre-compression condition after it moves out of the press part.
Compressibility and recoverability are necessary in a felt because, if the felt were not compressed when entering the press part of the papermaking machine, the wet paper web would be torn as a result of the pressure applied by the press rolls. Moreover, the speed of the felt and the press pressure have both increased as a result of developments in papermaking machinery in recent years. Accordingly, the conditions to which the felts are subject have become more severe, and it has been a challenge to produce a belt in which compression recovery and felt thickness are maintained so that felt has a satisfactory useful life.
Various proposals for structures which maintain compressibility and recoverability have been made.
One such proposal, described in Japanese Utility Model Registration No. 2514509, is a felt comprising a base fabric woven of thread, and a staple fiber integrated by needle punching with the base fabric. This felt uses fibers which exhibit elasticity as the threads of the base fabric or as the staple fiber. Fibers comprising a polyamide block copolymer which has hard segments composed of polyamide components and soft segments composed of polyether components, can be used as the elastic fibers.
On the other hand, for the purpose of improving compressibility and recoverability, a different felt structure, which does not comprise a base fabric and a staple fiber, has been proposed in Unexamined Japanese Patent Publication No. 504167/2001. In this felt, as shown in FIG. 18, a base body 20A comprises not just a woven fabric 20A1, but also a compact, mesh-shaped, thermoplastic resin sheet 20A2, and a multi-filament reinforcing yarn 20A3, the yarns being surrounded by a synthetic rubber material.
As shown in FIG. 19, another press felt has a layer of a three-dimensional knitted fabric, comprising two pieces of fabric 44A and 46A, and connecting fibers 48A connecting the two pieces of fabric. The connecting fibers 48A connect corresponding front and back stitches of the fabrics 44A and 46A, and these two pieces of fabric are supported by the connecting fibers 48A. Compression recoverability and the ability to maintain thickness can be improved by providing this three-dimensional knitted fabric in the felt, since, even when the three-dimensional knitted fabric is compressed under load, when the load is removed, the connecting fibers 48A recover their original form in the direction of the thickness of the three-dimensional fabric.
In the felt made in accordance with the first of the above-described proposals, recoverability diminished over repeated passage through the press part, due to the crushing of air voids formed between staple fibers.
In the case of the structure shown in FIG. 18, where an elastic structure, comprising a sheet 20A2 and reinforcement yarns 20A3, is used for improving the sustainability of the felt's thickness, the elastic structure is not compressed easily. As a result, its compression recoverability is not very different from that of the felt shown in FIG. 17, which has no elastic structure.
A press felt having a three-dimensional knitted fabric as shown in FIG. 19 exhibits improved compression recoverability and improved ability to maintain thickness to some extent. However, since the connecting fibers 48A, between the two pieces of fabric 44A and 46A, connect only corresponding front and back stitches of the respective pieces of fabric, the forces exerted on the connecting fibers during compression of the felt are exerted perpendicular to the stitch lines and tend to push all of the connecting fibers in the same direction. Consequently the elasticity of the press felt, its compression recoverability, and the ability of the felt to maintain its thickness are not entirely satisfactory. Furthermore if the connecting fibers are all pushed down in the direction of the width of the press felt, the press felt vibrates in the direction of the axes of the press rolls.
In view of the above problems, the principal object of this invention is to provide a papermaking press felt having superior compression recoverability and a superior ability to maintain its thickness. It is also an object of the invention to provide a method of manufacture of such a press felt.